Enhancement of secondary organic aerosol formation and its oxidation state by SO2 during photooxidation of 2-methoxyphenol

Enhancement of SOA yield and OSC from 2-methoxyphenol by SO2

Enhancement of secondary organic aerosol formation and its oxidation state by SO2 during photooxidation of 2-methoxyphenolEnhancement of SOA yield and OSC from 2-methoxyphenol by SO2Changgeng Liu et al.

2-Methoxyphenol (guaiacol) is derived from the lignin pyrolysis
and taken as a potential tracer for wood smoke emissions. In this work, the
effect of SO2 at atmospheric levels (0–56 ppbv) on secondary organic
aerosol (SOA) formation and its oxidation state during guaiacol
photooxidation was investigated in the presence of various inorganic seed
particles (i.e., NaCl and (NH4)2SO4). Without SO2 and seed
particles, SOA yields ranged from (9.46±1.71) % to (26.37±2.83) % and could be well expressed by a one-product model. According to
the ratio of the average gas-particle partitioning timescale (τ‾g-p) over the course of the experiment to the vapor wall deposition
timescale (τg−w), the determined SOA yields were
underestimated by a factor of ∼2. The presence of
SO2 resulted in enhancing SOA yield by 14.04 %–23.65 %. With
(NH4)2SO4 and NaCl seed particles, SOA yield was enhanced by
23.07 % and 29.57 %, respectively, which further increased
significantly to 29.78 %–53.43 % in the presence of SO2,
suggesting that SO2 and seed particles have a synergetic contribution to
SOA formation. The decreasing trend of the τ‾g-p/τg-w ratio in the presence of seed particles and SO2
suggested that more SOA-forming vapors partitioned into the particle phase,
consequently increasing SOA yields. It should be noted that SO2 was
found to be in favor of increasing the carbon oxidation state (OSC) of
SOA, indicating that the functionalization or the partitioning of
highly oxidized products into particles should be more dominant than
the oligomerization. In addition, the average N∕C ratio of SOA was
0.037, which revealed that NOx participated in the photooxidation
process, consequently leading to the formation of organic N-containing
compounds. The experimental results demonstrate the importance of SO2 on
the formation processes of SOA and organic S-containing compounds and are also
helpful to further understand SOA formation from the atmospheric
photooxidation of guaiacol and its subsequent impacts on air quality and
climate.

As a representative type of methoxyphenols, guaiacol mainly exists in the
gas phase and is widely found in the atmosphere (Schauer et al., 2001). Its
emission factor of wood burning is in the range of 172–279 mg kg−1
wood (Schauer et al., 2001). In recent years, the reactivity of gas-phase
guaiacol toward OH radicals (Coeur-Tourneur et al., 2010a), NO3
radicals (Lauraguais et al., 2016; Yang et al., 2016), Cl atoms (Lauraguais
et al., 2014a), and O3 (El Zein et al., 2015) has been investigated,
suggesting that its degradation by OH and NO3 radicals might be
predominant in the atmosphere. Meanwhile, several studies have reported the
significant secondary organic aerosol (SOA) formation from guaiacol oxidation by OH radicals, produced
from the photolysis of the OH precursors (i.e., H2O2 and
CH3ONO) (Ahmad et al., 2017; Lauraguais et al., 2014b; Yee et al.,
2013). However, SOA formation from the photooxidation of guaiacol in the
presence of NOx has not been investigated without adding a direct OH
precursor, even though it has been recently reported that the atmospheric
level of NOx could reach up to 200 ppbv in severely polluted
atmospheres of China (Li et al., 2017).

Although many studies concentrated on the SOA production from the oxidation
of volatile organic compounds (VOCs), the reported SOA yields showed high
variability for a given precursor (Chu et al., 2016, 2017; Ge et al., 2017a;
Lauraguais et al., 2012, 2014b; Ng et al., 2007; Sarrafzadeh et al., 2016;
Yee et al., 2013). This variability is mainly dependent on the numerous
factors, e.g., preexisting seed particles, SO2 level, NOx level,
humidity, and temperature. Two of the critical factors are the impacts of
preexisting seed particles and SO2 level on SOA formation (Chu et al.,
2016, 2017; Ge et al., 2017a). In addition, the atmospheric concentration of
SO2 could be close to 200 ppbv in severely polluted atmospheres of
China, and SOA from biomass burning and sulfate formation could
significantly contribute to severe haze pollution (Li et al., 2017). During
the transport process, smoke plumes from biomass burning would be inevitably
mixed with suspended particles (e.g., (NH4)2SO4 particles),
SO2, and NOx in the atmosphere. However, the influences of these
co-existing pollutants on the transformation of guaiacol and its SOA
formation are still unclear. For these reasons, the aim of this work was to
investigate the SOA formation from guaiacol photooxidation in the presence
of NOx in a 30 m3 indoor smog chamber, as well as the effect of
SO2 on SOA formation with various inorganic seed particles.

2.1 Smog chamber

The photooxidation experiments were performed in a 30 m3 indoor smog
chamber (4 m height × 2.5 m width × 3 m length), which
was built in a temperature-controlled room located at the Research Center for
Eco-Environment Sciences, Chinese Academy of Sciences
(RCEES-CAS). The details have been described elsewhere (Chen et al., 2019) and are shown in Fig. S1 in the Supplement. Briefly, 120 UV lamps
(365 nm, Philips TL 60/10R) were taken as the light source with a
NO2 photolysis rate of 0.55 min−1, which was comparable to
the irradiation intensity at noon in Beijing (Chou et al., 2011). A maglev
fan installed at the bottom center of the smog chamber was used to mix
sufficiently the introduced gas species and seed particles. Temperature (T)
and relatively humidity (RH) in the chamber were (302±1) K and (39±1) %, respectively. Before each experiment, the chamber was flushed
by purified dry zero air for ∼36 h with a flow rate of
100 L min−1 until the particle number concentration in the chamber was
lower than 20 cm−3.

2.2 Experimental procedures

Gas-phase guaiacol was firstly introduced into the chamber by purified dry
zero air flowing through the gently heated injector with a known volume of
pure liquid guaiacol until guaiacol fully vaporized. Its concentration in
the chamber was measured in real time by a high-resolution proton-transfer
reaction time-of-flight mass spectrometer (HR-ToF-PTRMS) (Ionicon Analytik
GmbH) and was calibrated by a commercial permeation tube (VICI AG
International Valco Instruments Co., Inc.). When guaiacol concentration was
stable, NO and SO2 were introduced into the chamber by a mass flow
meter using purified dry zero air as the carrier gas. Their concentrations
were controlled by the injection time preset through the electromagnetic
valve and were measured by a NOx analyzer (model 42i-TL, Thermo Fisher
Scientific, Inc.) and a SO2 analyzer (model 43i, Thermo Fisher
Scientific Inc.), respectively. In this work, the initial ratio (V∕V) of
guaiacol concentration to NOx concentration in the chamber was similar
in all experiments (∼1.2) (Tables 1 and 2). In addition,
sodium chloride (NaCl) and ammonium sulfate ((NH4)2SO4) were
used as the inorganic seeds. The seed aerosols in the chamber were generated
by the atomization of a 0.02 M aqueous solution. Through atomization, the
size distribution of seed particles peaked at 51–58 nm was achieved, with
a number concentration of 10 100–11 400 cm−3 (Table 2). After gas
species and seed particles in the chamber were mixed well, the
photooxidation experiment was carried out with the fan turned off. In this
work, the OH concentrations in the chamber were (1.3–2.2) × 106 molecules cm−3, calculated
based on the degradation rate (7.53×10-11 cm3 molecule−1 s−1) of guaiacol with OH
radicals (Coeur-Tourneur et al., 2010a). The chemicals and gas samples used
in this work were described in Supplement.

2.3 Data analysis

The HR-ToF-PTRMS with a time resolution of 1 min was used online to measure
the gas-phase concentration of guaiacol, and its m∕z range was 10–500 in the
process of data acquisition. Before data collection, the peaks of the
protonated water ([H318O]+) and protonated acetone
([C3H7O]+) ions at m/z=21.0246 and 59.0491 were used for mass
calibration, with the aim of obtaining accurate mass determination during
the experimental process. All data obtained by the HR-ToF-PTRMS were analyzed
with the PTR-MS Viewer software (version 3.1.0, IONICON Analytik).

An Aerodyne high-resolution time-of-flight aerosol mass spectrometer
(HR-ToF-AMS) was applied to online measure the chemical composition of
particles and the non-refractory submicron aerosol mass (DeCarlo et al.,
2006). For all experiments, the acquisition time of the HR-ToF-AMS was 2 min. The inlet flow rate, ionization efficiency, and particle sizing of the
HR-ToF-AMS were calibrated at regular intervals, according to the standard
protocols using the size-selected pure ammonium nitrate particles (Drewnick
et al., 2005; Jimenez et al., 2003). All data obtained by the HR-ToF-AMS
were analyzed by the ToF-AMS analysis toolkit SQUIRREL 1.57I/PIKA 1.16I
version, in Igor Pro version 6.37. The size distribution and concentration
of particles were monitored by a scanning mobility particle sizer (SMPS),
which is composed of a differential mobility analyzer (DMA) (model 3082, TSI
Inc.) and a condensation particle counter (CPC) (model 3776, TSI Inc.).
Assuming that particles are spherical and non-porous, the average particle
density could be calculated to be 1.4 g cm−3 using the equation ρ=dva/dm (DeCarlo et al., 2004), where dva is the mean vacuum
aerodynamic diameter measured by the HR-ToF-AMS and dm is the mean
volume-weighted mobility diameter measured by the SMPS. The mass
concentration of particles measured by the HR-ToF-AMS was corrected by the
SMPS data in this work using the same method as Gordon et al. (2014). In
this work, the wall loss rate (kdep) of (NH4)2SO4
particles could be expressed as kdep=4.15×10-7×Dp1.89+1.39×Dp-0.88 (Dp is
the particle diameter, nm), which was measured according to the literature
method (Takekawa et al., 2003) and was used to correct the wall loss of SOA.
In addition, its wall loss rate was determined at predetermined time
intervals, which only had a slight change among different experiments.

2.4 Vapor wall-loss correction

Previous studies have indicated that the losses of SOA-forming vapors to
chamber walls can result in the substantial and systematic underestimation of
SOA yield (Zhang et al., 2014, 2015). Therefore, SOA yields obtained in this work
were also corrected by vapor wall loss. The effect of vapor wall deposition
on SOA yields mainly depends on the competition between the uptake of
organic vapors by aerosol particles and the chamber wall (Zhang et al.,
2015). Thus, the ratio of the average gas-particle partitioning timescale
(τ‾g-p) over the course of the experiment to the vapor
wall deposition timescale (τg−w) could be reasonably used
to evaluate the underestimation of SOA yields. The detailed calculation of
τ‾g-p and τg−w was shown in the
Supplement.

3.1 SOA yields

A series of experiments were conducted at different guaiacol∕NOx
concentrations under atmospheric pressure. The experimental conditions and
results are shown in Table 1. SOA yield was calculated to be the ratio of
SOA mass concentration (Mo, µg m−3) to the consumed guaiacol
concentration (Δ[guaiacol], µg m−3) at the end of each
experiment (Kang et al., 2007). The results showed that SOA yield was
dependent on the initial guaiacol concentration ([Guaiacol]0). Higher
precursor concentration would result in a higher amount of condensable
products, subsequently enhancing SOA formation (Lauraguais et al., 2012). In
addition, it should be noted that SOA mass could directly affect the
gas-particle partitioning via acting as the adsorption medium of oxidation
products; thus, higher SOA mass generally leads to a higher SOA yield
(Lauraguais et al., 2014b).

Table 1Experimental conditions and results for guaiacol photooxidation in
the presence NOx.

a The consumed guaiacol concentration at the end of each
experiment.bMo is the mass concentration of SOA.

SOA yield (Y) could be represented by a widely used semi-empirical model
based on the absorptive gas-particle partitioning of semi-volatile products,
typically calculated using the following equation (Odum et al., 1996):

(1)Y=∑iMoαiKom,i1+Kom,iMo,

where αi is the mass-based stoichiometric coefficient for the
reaction producing the semi-volatile product i, Kom,i is the
gas-particle partitioning equilibrium constant, and Mo is the total
aerosol mass concentration.

Figure 1SOA yield as a function of SOA mass concentration (Mo) for
guaiacol photooxidation in the presence of NOx at different guaiacol
concentrations. The lines were fit to the experimental data using a
one-product model. Values of α and Kom,i used to generate the
solid line were (0.27±0.01) and (0.033±0.008), and their
values for the dashed line were (0.52±0.03) and (0.025±0.006),
respectively.

The yield curve for guaiacol photooxidation is shown in Fig. 1, obtained by
plotting the SOA yield data in Table 1 according to Eq. (1). The yield data
were accurately reproduced by a one-product model (R2=0.97), while
two or more products used in the model did not significantly improve the
fitting quality. The obtained values of αi and Kom,i for
one-product model were (0.27±0.01) and (0.033±0.008) m3µg−1, respectively. In previous studies, the one-product model was
widely applied to describe SOA yields from the oxidation of aromatic
compounds including methoxyphenols (Coeur-Tourneur et al., 2010b; Lauraguais
et al., 2012, 2014b). In this work, this simulation suggests that the
products in SOA have similar values of αi and Kom,i, i.e.,
the obtained αi and Kom,i are the average values. The plot
shown in Fig. S2 is the relationship between Mo versus Δ[guaiacol], of
which slope (0.28) is slightly higher than αi value (0.27).
This suggests that the formed low-volatile products almost completely
partitioned into the particle-phase according to the theoretical partition
model (Lauraguais et al., 2012, 2014b).

In the previous studies, the significant SOA formation from the OH-initiated
reaction of guaiacol has been reported (Lauraguais et al., 2014b; Yee et
al., 2013). In this work, SOA yields for guaiacol photooxidation range from
(9.46±1.71) % to (26.37±2.83) %, shown in Table 1.
According to the ratios of τ‾g-p/τg-w
(0.61–0.93), the determined SOA yields were underestimated by a factor of
∼2, suggesting that vapor wall loss in the chamber
could significantly affect SOA formation. The similar results were reported
previously by Zhang et al. (2014), who observed that SOA yields for toluene
photooxidation were substantially underestimated by factors of as much as 4, caused by vapor wall loss. As shown in Fig. 1, the vapor wall-loss-corrected SOA yields were in the range of (15.24±0.85) % to
(50.89±2.87) %, and could also be reproduced by a one-product
model (R2=0.96). This range overlaps SOA yields of 0.6 %–87 % for guaiacol
oxidation under high NOx condition (∼10 ppmv NO), reported by Lauraguais et al. (2014b), using CH3ONO as the
OH source. Under low NOx conditions (<5 ppbv NO), SOA yields
for guaiacol oxidation were in the range of 44 %–50 %, reported by
Yee et al. (2013) using H2O2 as the OH source and
(NH4)2SO4 as seed particles; they also indicated that high
NOx concentration (>200 ppbv NO) played an opposite role in
SOA formation. Overall, the vapor wall-loss-corrected SOA yields in this
work are well in agreement with those reported previously (Lauraguais et
al., 2014b; Yee et al., 2013), but the determined SOA yields are much lower.
Therefore, the effect of vapor wall loss on SOA formation should be
seriously taken into account.

In addition, the average N∕C ratio of SOA for guaiacol photooxidation in the
presence of NOx is 0.037, calculated according to the element analysis
by the HR-ToF-AMS. This indicates that NOx participates in SOA formation and growth. This phenomenon is well supported by the previous studies,
which have reported that the nitro-substituted products are the main
products of the OH-initiated reaction of guaiacol in the presence of
NOx (Ahmad et al., 2017; Lauraguais et al., 2014b). The relative low
volatility of these products could reasonably contribute to SOA formation
(Duporté et al., 2016; Liu et al., 2016a). The average
NO+/NO2+ ratio of SOA from guaiacol photooxidation
is 4.08, which is higher than that (2.06–2.54) for ammonium nitrate,
determined by the HR-ToF-AMS in this work. The possible explanation might be
that nitro-organics and organonitrates both exist in SOA (Farmer et al.,
2010; Sato et al., 2010). The relative abundance of organic N-containing
compounds could be estimated from the average N∕C ratio. Assuming that the
oxidation products in the SOA retain seven carbon atoms, the yield of organic
N-containing compounds is 25.9 %, which is the upper limit due to the
possible C–C bond scission during photooxidation process.

3.2 Effect of SO2 on SOA formation

3.2.1 SOA yields

In China, atmospheric SO2 concentration is always in the range of
several to dozens of parts per billion by volume, while in the severely polluted atmosphere it
could be up to close 200 ppbv (Han et al., 2015; Li et al., 2017). In
addition, a recent field measurement study has reported that the decrease in
biogenic SOA mass concentration in the atmosphere has a positive correlation
with SO2 emission controls (Marais et al., 2017). Therefore, the effect
of SO2 at atmospheric levels on SOA formation from guaiacol
photooxidation under atmospheric NOx conditions was investigated. The
experimental conditions and results are shown in Table 2. The formation of
SOA, sulfate, and nitrate as a function of SO2 concentration for
guaiacol photooxidation is shown in Fig. S3, and the time-series variations
in the concentrations of sulfate and nitrate are shown in Fig. S4. The
decays of guaiacol, NOx, and SO2 are shown in Figs. S5a, S6a, and
S7, respectively, which have similar changing trends for different
experiments. As illustrated in Fig. 2, the induction period became shorter
with the increase in SO2 concentration. The similar results caused by
SO2 have also been reported previously (Chu et al., 2016; Liu et al.,
2016b). Meanwhile, Mo for the experiment without SO2 (Exp. 1 in
Table 2) increased from (63.62±1.71) to (71.88±1.43) and
(78.59±2.06) µg m−3, enhanced by 12.98 % and
23.53 %, respectively, when SO2 concentration raised from 0 to 33 and
56 ppbv. The corresponding SOA yields were (21.60±1.27) % and (23.42±1.80) %, respectively. The similar results were reported by
previous studies (Kleindienst et al., 2006; Lin et al., 2013; Liu et al.,
2016b), which observed the significant enhancement of SOA yields for VOC
oxidation and the photochemical aging of gasoline vehicle exhaust in the
presence of SO2.

Table 2Experimental conditions and results for guaiacol photooxidation in
the presence of seed particles and SO2.

a The consumed guaiacol concentration at the end of each experiment.
bNS is the initial seed number.
cDS is the average
diameter of seed particles.
dCseed is the initial concentration of seed.
eCsulfate is the sulfate concentration formed by SO2
oxidation.f The initial surface area of seed particles.
g The final surface
area of aerosol particles (seed + organic aerosol), measured by the
SMPS.
h The ratio of the average gas-particle partitioning timescale
(τ‾g-p) over the course of the experiment to thevapor
wall deposition timescale (τg−w).
iMo is the
mass concentration of SOA.
j OSC is the average oxidation state of
carbon of SOA.

As shown in Fig. 3, the τ‾g-p/τg-w ratio
decreased from 0.82 to 0.71 and 0.61 when the SO2 concentration increased
from 0 to 33 and 56 ppbv. It suggests that the sulfate formed via SO2
oxidation could serve as seed particles (Jaoui et al., 2008) and increase
the surface areas of particles (Xu et al., 2016). These roles are favorable
to partitioning more SOA-forming vapors into the particle phase (Zhang et al.,
2014), consequently enhancing SOA yields. At the same time, as shown in Fig. S4 and Table 2, the sulfate concentration increased significantly from 7.42
to 17.89 µg m−3 when SO2 concentration increased from 33 to
56 ppbv. Nevertheless, the particle peak attributed to sulfate formed via
SO2 oxidation was not observed by the SMPS during the experimental process
due to the quick particle growth in the presence of organic vapors. In this
work, it is difficult to completely remove traces of NH3 from zero
air; thus, the formed sulfate should be the mixture of H2SO4 and
(NH4)2SO4. The time-series changes in the concentration of
ammonium salt at different SO2 concentrations are shown in Fig. S8. Its
concentration increased obviously with increasing SO2 concentration,
suggesting that more (NH4)2SO4 was produced.
Similar results have also been reported recently by Chu et al. (2016).
In addition, the surface area concentration of aerosol particles at the end
time was calculated. As shown in Table 2, the final surface area of aerosol
particles formed via guaiacol photooxidation increased from 1.25×103 to 1.68×103 and 2.04×103µm2 cm−3
when SO2 concentration increased from 0 to 33 and 56 ppbv. The increased surface area could be in favor of outcompeting the wall
loss for low-volatility vapors produced from guaiacol photooxidation; i.e.,
more low-volatility vapors would be diverted from wall loss to the
particles, consequently increasing SOA yields (Kroll et al., 2007). This is
well supported by the decrease in the τ‾g-p/τg-w ratio with increasing SO2 concentration, shown in Fig. 3.

Figure 3Variations in the τ‾g-p/τg-w
ratio in the presence of various seed particles as a function of SO2
concentration.

The time-series changes in the mass concentrations of NO+
and NO2+ are shown in Fig. S9a. The mass concentration of
NO+ increased more quickly than that of
NO2+ and had a positive correlation with SO2
concentration. But compared to the experiment without SO2, the
presence of SO2 had little impact on NO+/NO2+
and N∕C ratios obtained at the end time, shown in Figs. S9b and S10b,
respectively. These ratios indicated that organic N-containing compounds
were also produced in this system (Farmer et al., 2010; Sato et al., 2010).

3.2.2 Oxidation state of SOA

The average carbon oxidation state (OSC=2O/C-H/C) of OA is
widely used to represent the oxidation degree of atmospheric OA, because it
takes into account the saturation level of carbon atoms in the OA (Kroll et
al., 2011). As shown in Table 2, increasing SO2 concentration
(0–56 ppbv, Exps. 1–3) leads to the increase in OSC (0.11–0.18). The
variations in H∕C, O∕C, and N∕C ratios as a function of irradiation time are
shown in Fig. S10. In order to further identify the effect of SO2 on
the chemical properties of SOA, positive matrix factorization (PMF) analysis
for all AMS data obtained at different SO2 concentrations over the
course of the experiments was carried out. Two factors were obtained from the
PMF analysis, and their mass spectra are shown in Fig. 4. The organic mass
fraction of m/z=44 (CO2+), named f44, was 0.122 for Factor 2, which is higher than that (0.094) for Factor 1. Therefore, Factor 2 was
tentatively assigned to the more-oxidized SOA, while Factor 1 was the
less-oxidized SOA (Ulbrich et al., 2009). During the photooxidation process,
these two factors had different variations as a function of irradiation
time. As shown in Fig. S11, Factor 1 increased along with the reaction and
then decreased, while Factor 2 had an increasing trend. Compared to Exps. 1
and 2 in Table 2, the higher fraction of Factor 2 mass obtained at 56 ppbv
SO2 (Exp. 3 in Table 2) suggests that the formed SOA mainly consists of
more-oxidized products with relatively low volatility. This is well
supported by the time-series variations in the fraction of organic ion
groups (CH+, CHO+, and CHOgt1+ – containing
more than one oxygen atom) (Fig. S12a), which shows the higher fraction of
CHOgt1+ and lower fraction of CH+ obtained at
higher SO2 concentration, consequently resulting in a higher OSC of
SOA.

Figure 4Mass spectra of Factor 1 (a) and Factor 2 (b) for the formed SOA
identified by applying PMF analysis to the AMS data, obtained at different
SO2 concentrations over the course of the experiments.

Previous studies mostly reported that the enhancement of SOA yield in the
presence of SO2 was ascribed to the functionalization and
oligomerization reactions (Cao and Jang, 2007; Jaoui et al., 2008; Liu et
al., 2016b; Xu et al., 2016). If the oligomerization reaction plays a
predominant role in the presence of SO2 which will lead to particle-phase H2SO4, the carbon number of oligomers will increase but
their net O∕C or H∕C values will show little change, consequently resulting in
little change in the oxidation state of SOA (Chen et al., 2011).
Nevertheless, we observed that SO2 not only enhanced SOA yields, but
also resulted in higher OSC (Table 2 and Fig. 5). This suggests that
the functionalization reaction might be predominant with SO2, which
leads to higher OSC of products with low molecular weight (MW) (Ye et
al., 2018), consequently resulting in an overall increase in OSC and
SOA yields. More recently, Ye et al. (2018) also found the similar results
in the ozonolysis of limonene. Figure S13 shows the differences among the
normalized mass spectra of SOA formed at different SO2 concentrations.
As shown in Fig. S13a, the signal fractions from the low-MW species were
enhanced significantly in the presence of SO2 and were much higher
than those from the high-MW species (m∕z>300). The similar results
were also observed in Fig. S13b when increasing SO2 concentration. In
other words, SO2 played a more important role in the formation of
organic S-containing compounds and the formation or uptake of low-MW
species, compared to the formation of high-MW species (i.e., oligomers) that
should be reasonably produced via the acid-catalyzed heterogeneous reactions
(Cao and Jang, 2007; Jaoui et al., 2008; Liu et al., 2016b; Xu et al.,
2016).

Figure 5OSC of SOA formed in the presence of various seed particles
as a function of SO2 concentration.

In this work, assuming that all organic S-containing compounds are
organosulfates and have the same response factor and fragmentation as methyl
sulfate, the conservative lower-bound concentration of organosulfates was
calculated to be in the range of (2.1±0.8) to (4.3±1.7) ng m−3 using the method described by Huang et al. (2015) shown in the
Supplement and increased with the increase in SO2 concentration. This
concentration range is close to those derived from the atmospheric oxidation
of polycyclic aromatic hydrocarbons and alkane (Meade et al., 2016; Riva et
al., 2015). Figure S14 is the examples of the ions (i.e., CSO+,
CH3SO2+, and CH3SO3+) of methyl sulfate obtained at 56 ppbv SO2 (Exp. 3
in Table 2). On the other hand, sulfuric acid formed from SO2 may be
favorable for the uptake of water-soluble low-MW species (e.g., small
carboxylic acids and aldehydes) and also be helpful for retaining them in
the aerosol phase, which would result in the increase in OSC. This is well
supported by the time-series variations in the concentrations of acetic acid
at different SO2 concentrations measured by the HR-ToF-PTRMS (Fig. S15a), which shows that acetic acid concentration decreased with the
increase in SO2 concentration (0–56 ppbv). These results were in good
agreement with those reported by Liggio et al. (2005) and Liu et al. (2010),
who observed that the uptake of organic compounds under acidic conditions
would be enhanced significantly. Recently, Huang et al. (2016) have also
reported that acetic acid is present in SOA formed via α-pinene
ozonolysis and its uptake would increase in the presence of seed particles.
In addition, Krapf et al. (2016) have indicated that peroxides in SOA are
unstable and liable to decompose into volatile compounds, consequently
leading to decreases in SOA yield and OSC. But Ye et al. (2018) found that
the reactions of SO2 with organic peroxides were the dominant sink of
SO2, initiated by the heterogeneous uptake of SO2 under humidity
conditions. These reactions would result in the formation of organic
S-containing compounds, consequently increasing SOA yields and OSC.

3.3 Effect of inorganic seed particles on SOA formation

Seed particles are one of the critical factors influencing SOA formation (Ge
et al., 2017a); thus, the effects of inorganic seeds (i.e., NaCl and
(NH4)2SO4) on SOA formation from guaiacol photooxidation were
investigated. As shown in Fig. 6, the presence of inorganic seed particles
could accelerate the SOA growth rate at the initial stage of photooxidation
(i.e., it would shorten the induction period), followed by the decrease in the growth rate
along with the reaction, because the presence of inorganic seeds could
promote the condensation of SOA-forming organic products and consequently
increase SOA formation (Yee et al., 2013). The results showed that Mo
for the experiment without seed particles (Exp. 1 in Table 2) increased from
(63.62±1.71) to (79.44±1.86) and (84.91±2.01) µg m−3 (Table 2), enhanced by 24.87 % and 33.46 %, respectively,
with (NH4)2SO4 and NaCl seed particles. The corresponding SOA
yields were (23.31±1.59) % and (24.54±1.73) %,
respectively. In previous work, the similar results on the enhancements
of SOA formation by NaCl and (NH4)2SO4 seed particles were
reported in the oxidation of VOCs (Ge et al., 2017a, b; Huang et al.,
2013, 2017). As shown in Fig. 3, τ‾g-p/τg-w ratios with (NH4)2SO4 and NaCl seed particles
were 0.62 and 0.54, respectively, which suggested that more SOA-forming
vapors partitioned into the particle phase in the presence of NaCl seed
particles (Zhang et al., 2014), consequently resulting in a relatively higher
SOA yield.

As shown in Table 2 and Fig. 6, the SOA mass concentration in the presence
of NaCl seed particles was higher than that in the presence of
(NH4)2SO4 seed particles. In addition, the OSC of SOA in the
presence of NaCl seed particles is 0.29, slightly higher than that (0.20) in
the presence of (NH4)2SO4 seed particles. Recently, it has
been also reported that the presence of (NH4)2SO4 and
NaNO3 seed particles could significantly enhance the oxidation state of
SOA, compared to without seed particles (Huang et al., 2016). In this work,
the experimental conditions for seed experiments are almost the same (Table 2), including reactant concentration, temperature, RH, and the number and
diameter of seed particles. Therefore, the differences in the yield and
oxidation state of SOA resulted from the different chemical
compositions of SOA in the presence of different inorganic seeds. As shown
in Fig. S12b and c, compared to (NH4)2SO4 seed particles,
the higher fraction of CHOgt1+ and lower fraction of
CH+ were obtained with NaCl seed particles, consequently resulting in
higher OSC of SOA. The time-series evolution of O∕C, H∕C, and N∕C
ratios is shown in Figs. S16 and S17, which indicate that O∕C ratios
(0.94–0.99) with NaCl seed particles at the end of experiments are higher
than those (0.90–0.93) with (NH4)2SO4 seed particles. Figure 7 shows the mass spectra of SOA in the presence of NaCl and
(NH4)2SO4 seed particles obtained by the HR-ToF-AMS, as well
as their difference mass spectrum. As shown in Fig. 7, f44 for SOA in
the presence of NaCl seed particles was higher than that obtained in the
presence of (NH4)2SO4 seed particles, while the mass
fractions of m/z=15 (CH3) and 29 (CHO) fragments were both lower. The
m/z=44 ion (CO2+) is mainly contributed from acids or
acid-derived species, such as esters (Ng et al., 2011). The higher
f44 of SOA with NaCl than (NH4)2SO4 seed particles
suggests that the distribution of highly oxidized small carboxylic acids
onto seed particles plays an important role in SOA formation, consequently
resulting in a higher oxidation state of SOA (Huang et al., 2016; Ng et al.,
2011). Compared to (NH4)2SO4, the hygroscopicity of NaCl is
stronger (Ge et al., 2017a; Gysel et al., 2002). The molar ratio of H2O
to NaCl is about 0.1 at 40 % RH, and water is mainly adsorbed on NaCl
particles (Weis and Ewing, 1999). Thus, the greater water content on the
particle surface could facilitate the uptake of highly oxidized small
carboxylic acids onto NaCl particles, which might explain the higher SOA
oxidation state observed in the presence of NaCl seed particles (Huang et
al., 2016). As shown in Fig. S15, the concentration of acetic acid in the
gas phase with NaCl seed particles was lower than that with
(NH4)2SO4 seed particles. It suggests that the uptake of
acetic acid on NaCl seed particles might be higher than that on
(NH4)2SO4 seed particles under similar experimental
conditions (i.e., NOx and guaiacol concentrations, temperature, and
RH). Moreover, the adsorbed acid products would also generate H+ ions,
which could catalyze heterogeneous reactions to produce more-oxidized
products or oligomers with relatively low volatility (Fig. S18),
consequently resulting in the enhancement of SOA formation (Huang et al.,
2013, 2017; Cao and Jang, 2007; Jaoui et al., 2008; Liu et al., 2016b; Xu et
al., 2016).

Figure 7Mass spectra of SOA with NaCl (a) and (NH4)2SO4(b)
as seed particles obtained by the HR-ToF-AMS, as well as their difference
mass spectrum (c=a-b).

In addition, the possible formation of Cl atoms from the photolysis of
nitryl chloride (ClNO2⟶hυCl+NO2,
k1=∼10-4 s−1) (Mielke et al., 2011) and the reaction of OH
radicals with Cl− (Cl-+OH→Cl+OH-,
k2=∼109 M−1 s−1) (Fang et al., 2014)
would also initiate a series of reactions to oxidize SOA composition, which
might be another reason for higher OSC observed with NaCl seed
particles. According to the rate constant (109 M−1 s−1) (Fang
et al., 2014), the uptake coefficient (3.4×10-3) of OH
radicals on NaCl particles (Park et al., 2008), and the concentrations of OH
radicals and Cl−, the concentration of Cl atoms produced from the
reaction of OH radical with Cl− was estimated to be less than 38 molecules cm−3, which was much higher than that from the photolysis of
ClNO2 due to the slow photolysis rate constant of
∼10-4 s−1 (Mielke et al., 2011). Compared to OH
concentration in the chamber, the oxidation of SOA composition by Cl atoms
should be insignificant.

According to the former results obtained in this work, it is known
that SO2 and inorganic seed particles both have a positive role in
enhancing SOA formation. Therefore, their possible synergetic effects on SOA
formation were investigated. Considering the experiments performed under the
comparable conditions (Table 2), the results should be reasonably reliable.
The decays of guaiacol, NOx, and SO2 are shown in Figs. S5, S6,
and S7, respectively, which have the similar changing trends for different
experiments. Figure S19 shows the time-series evolution in the sulfate
concentration in the presence of different SO2 concentrations and seed
particles, which indicates that sulfate concentration is dependent on
SO2 concentration. As shown in Fig. 8, the addition of SO2 into
the chamber in the presence of inorganic seed particles significantly
promoted SOA formation from guaiacol photooxidation but had an ignorable
impact on the induction period. When SO2 concentration raised from 0 to
30 and 54 ppbv in the presence of NaCl seed particles, Mo increased
from (84.91±2.01) to (90.89±2.28) and (98.86±2.11) µg m−3, enhanced by 7.04 % and 16.43 %, respectively, and
the corresponding SOA yields were (26.78±1.97) % and (29.06±1.82) %. For (NH4)2SO4 seed particles, Mo
increased from (79.44±1.86) to (84.35±2.09) for 33 ppbv
SO2 and (89.92±2.31) µg m−3 for 54 ppbv SO2,
enhanced by 6.18 % and 13.19 %, respectively, and the corresponding
SOA yields were (24.58±1.78) % and (26.37±1.98) %. As
shown in Fig. 3, the τ‾g-p/τg-w ratio had
a decreasing trend when increasing SO2 concentration in the presence of
seed particles, suggesting that the underestimation of SOA yields caused by
vapor wall loss was weakened significantly because of the additional sulfate
formed from SO2 oxidation. Thus, inorganic seed particles and SO2
showed a synergetic effect on SOA formation.

As shown in Table 2 and Fig. 5, it should be noted that OSC of SOA
increased in the presence of SO2, which was well supported by the
time-series variations in H∕C, O∕C, and N∕C ratios at different SO2
concentrations with NaCl and (NH4)2SO4 as seed particles,
shown in Figs. S16 and S17. In addition, as shown in Fig. S12b and c,
the higher fraction of CHOgt1+ and lower fraction of
CH+ were obtained at higher SO2 concentration, consequently
resulting in higher OSC of SOA. Figure S20 shows the mass spectra of SOA
with NaCl and (NH4)2SO4 as seed particles at different
SO2 concentrations obtained by the HR-ToF-AMS. As illustrated in Fig. S20, SO2 addition was in favor of increasing the value of f44,
suggesting that more products with higher OSC are produced by the
functionalization reaction (Ye et al., 2018). Meanwhile, Table 2 shows that
the final surface area of aerosol particles increased in the presence of
SO2, which played a positive role in diverting more low-volatility
vapors from wall loss to the particles, consequently enhancing SOA yields
(Kroll et al., 2007). In addition, the presence of inorganic seeds could
promote the condensation of SOA-forming organic products and the
heterogeneous uptake of SO2 (Yee et al., 2013), providing favorable
conditions for the following reactions. Meanwhile, the higher hygroscopicity
of NaCl than (NH4)2SO4 might be helpful to dissolve more acid
substances on NaCl particle surface (e.g., H2SO4 and organic
acid), especially in the presence of SO2. This hypothesis could be
supported by the variations in acetic acid concentration in the presence of
different seed particles and SO2 concentrations (Fig. S15), which shows
that acetic acid concentration decreased with the increase in SO2
concentration (0–54 ppbv). The dissolved acid compounds might be helpful
to catalyze heterogeneous reactions (Cao and Jang, 2007; Huang et al., 2013,
2017; Jaoui et al., 2008; Liu et al., 2016b; Xu et al., 2016). Figures S21 and
S22 show the differences among the normalized mass spectra of SOA formed at
different SO2 concentrations with various seed particles. The results
indicated that the signal fractions from the low-MW species increased
significantly in the presence of SO2 and were much higher than those
from the high-MW species (m∕z>300). Compared to Exps. 2 and 3 in
Table 2 with no seed particles, the conservative lower-bound concentrations
of organosulfates formed with seed particles were similar and in the range
of (2.2±0.7) to (4.6±1.8) ng m−3, which might be caused
by the similar SO2 concentrations applied for experiments. With NaCl
and (NH4)2SO4 as seed particles, SOA yields and OSC both
increased with the increase in SO2, suggesting that the
functionalization reaction should be more dominant than the oligomerization
reaction during photooxidation process.

In this work, SOA formation from guaiacol photooxidation in the presence of
NOx was investigated in a 30 m3 smog chamber. SOA yields for
guaiacol photooxidation were in the range of (9.46±1.71) % to
(26.37±2.83) % and could be expressed well by a one-product
model. These yields were underestimated by a factor of ∼2
according to τ‾g-p/τg-w ratios.
The presence of SO2 could increase SOA yield and OSC, indicating
that the functionalization reaction should be more dominant than
the oligomerization reaction. Meanwhile, the similar effect of SO2 was also
observed with NaCl and (NH4)2SO4 seed particles. But SOA
yield and OSC in the presence of NaCl seed particles were both slightly
higher than those in the presence of (NH4)2SO4 seed
particles. In addition, the results indicated the synergetic contribution of
SO2 and inorganic seed particles to SOA formation. The decreasing trend
of τ‾g-p/τg-w ratio in the presence
of seed particles and SO2 suggested that more SOA-forming vapors
partitioned into the particle phase, consequently increasing SOA yields. The
average N∕C ratio (0.037) of SOA suggested that NOx participated in the
process of guaiacol photooxidation, resulting in the formation of organic
N-containing compounds.

The significant SOA formation from guaiacol photooxidation at the
atmospheric levels of SO2 and NOx in this work suggests that more
attention needs to be given to the SOA formation from biomass burning and its
subsequent effects on haze evolution, especially in China with nationwide
biomass burning, because recent studies have indicated that SOA formed from
biomass burning plays an important role in haze pollution in China (Ding et
al., 2017; Li et al., 2017). In addition, the results imply that the
oxidation of SO2 and VOCs are tightly coupled, and SO2 has a
direct impact on the physics and chemistry of SOA formation. Although
guaiacol concentrations in the chamber study are higher than those in the
ambient atmosphere, the results obtained in this work could provide new
information for SOA formation from the photooxidation of methoxyphenols and
might be useful for SOA modeling, especially for air quality simulation
modeling of the specific regions experiencing serious pollution caused by
fine particulate matter. In addition, the results would help to further
understand the photochemical aging process of smoke plumes from biomass
burning in the atmosphere.

CL and TC contributed equally to this work and should be
considered as co-first authors.
CL, TC, YL, and HH designed the research and wrote the paper. CL, TC, and JL
performed the experiments. CL, TC, YL, JL, HH, and PZ carried out the data
analysis. All authors contributed to the final paper.

This work was financially supported by the National Key R&D Program of
China (2016YFC0202700), National Natural Science Foundation of China
(21607088 and 41877306), China Postdoctoral Science Foundation funded
project (2017M620071), Applied Basic Research Project of Science and
Technology Department of Sichuan Province (2018JY0303), and Key Research
Program of Frontier Sciences, CAS (QYZDB-SSW-DQC018). Yongchun Liu would like to
thank Beijing University of Chemical Technology for financial support.
The authors would also like to acknowledge the experimental help provided by Xiaolei Bao from Hebei Provincial Academy of Environmental Sciences, Shijiazhuang,
China.

The effect of SO2 at atmospheric levels on SOA formation and its oxidation state during 2-methoxyphenol photooxidation was investigated with various inorganic seed particles. The presence of SO2 increased SOA yield and oxidation state, suggesting that the functionalization reaction should be more dominant than the oligomerization reaction. SO2 and seed particles were found to have a synergetic contribution to SOA formation. The results demonstrate the important role of SO2 in SOA formation.

The effect of SO2 at atmospheric levels on SOA formation and its oxidation state during...